Sleep is regulated by a circadian process that modulates the timing of sleep and a homeostatic process that adjusts the amount and depth of sleep in response to sleep need. Studies over the past few decades have delineated many of the molecular mechanisms underlying the core circadian clock. However, the mechanisms by which this core clock regulates sleep remain poorly understood. Recently, using Drosophila as a gene discovery system, we identified a novel molecule named WIDE AWAKE (WAKE) that mediates the circadian timing of sleep onset. WAKE is expressed in arousal-promoting clock neurons and upregulates GABA signaling in a time-dependent manner to promote sleep in Drosophila. Strikingly, we find a single homolog of WAKE in mice (mWAKE) and have determined that mWAKE is specifically enriched in the suprachiasmatic nucleus (SCN), the master circadian pacemaker in mammals. The overall goal of this proposal is to characterize the molecular and cellular mechanisms underlying the circadian timing of sleep, by investigating the function of mWAKE in mice. To do this, we will employ an array of approaches, including molecular, biochemical, genetic, electrophysiological, and behavioral studies. Specifically, we propose to 1) determine the molecular mechanisms underlying mWAKE function in cultured cells and SCN slices, 2) examine the sleep and circadian phenotypes of mice genetically lacking mWAKE, and 3) characterize and identify the function of specific mWAKE circuits in mice. These studies will be carried out with an outstanding collaborative team, with expertise in mouse genetics, large-scale in situ hybridization experiments, SCN slice physiology, and circadian/sleep behavioral analyses and should yield new insights into SCN function and the molecular and cellular pathways mediating the timing of sleep. ~15 million Americans have to work alternate shift schedules, and emerging evidence suggests that dysregulated sleep/wake cycles in humans can have significant adverse health consequences. Thus, there is an increased urgency to understand the circadian timing mechanisms underlying sleep. The proposed studies should lead to a better understanding of these mechanisms and thus may facilitate the potential development of novel therapeutic targets for the treatment of circadian sleep/wake desynchrony.